Known braking pressure generating apparatus are disclosed in U.S. Pat. No. 4,126,996 published on Nov. 28, 1978 and U.S. Pat. No. 4,603,918 published on Aug. 5, 1986. Referring to the drawing figure in U.S. Pat. No. 4,126,996, the disclosed braking pressure generating apparatus includes a hydraulic pressure generator (54, 55) which generates and outputs a power pressure irrespective of operation of a brake pedal (2), and an input member (4) receiving the braking operation force through the brake pedal. A regulation valve (7) is disposed coaxially with the input member to regulate the power pressure outputted from the hydraulic pressure generator (54, 55) to a pressure corresponding to the braking operation force. To permit the input member stroke to correspond to the braking operation force, a travel spring (5) is disposed between the input member and a valve portion of the regulation valve so as to be compressed between the input member and the valve portion.
As shown in FIGS. 1 and 6 of U.S. Pat. No. 4,603,918, the disclosed braking pressure generating apparatus includes a hydraulic pressure generator (24, 25) which generates a power pressure irrespective of operation of a brake pedal (16) and an input member (79) which receives the braking operation force through the brake pedal. A regulation valve (71) is disposed non-coaxially with the input member to regulate the power pressure outputted from the hydraulic pressure generator to a pressure corresponding to the brake operation force. In order to permit the input member stroke to correspond to the braking operation force, a first spring (72) is disposed between the input member and a valve portion (74) of the regulation valve so as to be compressed between the input member and the valve portion and a second spring (FIG. 6) is disposed between the input member and a flange portion of a body (FIG. 6) so as to be compressed between the input member and the flange portion.
The relationship between the braking operation force and the output pressure of the regulation valve differs depending upon the vehicle model for which the braking pressure generation apparatus is applied. To change this relationship in the case of the apparatus described in U.S. Pat. No. 4,126,996, the diameter of the valve portion receiving the output pressure of the regulation valve (7) must be changed. In addition, the corresponding inner diameter of the valve body must be also changed. Such a change can involve significant costs.
In the case of the apparatus disclosed in U.S. Pat. No. 4,603,918, the braking operation force can be distributed to the valve portion and the body of the regulation valve by the first and second springs. Thus, the distribution rate, i.e., the relationship between the braking operation force and the output pressure of the regulation valve, can be changed by changing the characteristics (e.g., spring constant) of the first spring and/or the second spring. While such a change may not be as costly as that associated with the apparatus described in U.S. Pat. No. 4,126,996, other difficulties may arise. For example, because the two springs function as both the stroke simulator and the braking operation force distribution device, if the distribution characteristics are changed by changing the first and/or the second springs, the stroke simulator characteristics (i.e., the relationship between the braking operation force and the stroke of the input member) are varied. As a result, it is difficult to independently set both the distribution characteristics and the stroke simulator characteristics to the appropriate or desired characteristics.
A need thus exists for a vehicle braking pressure generation apparatus which is not as susceptible to the drawbacks and disadvantages discussed above.